Blended process chemicals are required, for example, in the semiconductor industry. In typical blend style systems, there are two styles of mixing, which dominate the supply system marketplace: volumetric and mass. Both are quantitative techniques, which assume that the incoming liquids are uniform streams, within a narrow range of variance to chemical concentrations based on an assay of percentage of mass of specific chemical content by total unit mass. In most cases, these small fluctuations are of a few percentage; for example, semiconductor grade hydrogen peroxide, which is typically assumed to be at thirty percent by weight, may vary from 28 to 33 percent by weight. When targeting blends requiring final contributions by these chemicals to be from a few tenths of a percent to around, say, four percent by weight, the wide range of incoming percent weight of incoming chemical may lead to variations requiring redress to the final solution. Redress of a final solution may result form off-line or in-line techniques, which range from physical sample collection and assay via lab analysis to sample diversion and analysis by automated sample assay systems. A significant amount of time may result in the final acceptance of a liquid chemical blend, awaiting transfer, for use in a production environment.
The current style of blending liquids in a chemical application involves the use of volumetric or mass measurements to dispense each of the required constituents. Volumetric measurements are made through the use of a flow meter or level sensing. Flow meters measure the volume of liquid through some mechanism, such as paddle wheel, turbine wheel, ultrasonic frequency, differential pressure measurement, etc. Level sensors gauge the height of a column of liquid within the vessel and correlate the volume through a known cross sectional area of the vessel.
Volumetric measurements utilize flow meters, of various styles, and level measurement components to gauge a volume dispensed into a vessel. The volumes are delivered over a period of time, which may fluctuate accordingly based on the required input parameters of the individual liquid streams to dispense the correct amount of liquid. Input parameters are based on the support systems, which are typically pressure-supplied and could by subject to changes in flow rates supplied to the blending systems. Fluctuations may be caused by pulsations in the style of delivery mechanism used in supply equipment, from frictional losses incurred through support lines, or various head loss depending on the location of the supply equipment within a chemical processing facility. Any of these various contributions to slowing the supply flow of raw, individual supply streams creates an increase in time to dispense the chemical within a blend vessel.
Each of these styles of measurement incurs some form of error from the measurement. This error could be in the form of a set percentage, based on the level of sensitivity built into the device. The percentage may be a known, constant value, which is applied to the entire operational range of the flow meter. This is sometimes referred to as a full scale. Another form of error may be a percentage of the reading, which increases with increasing flow. As an example: a flow meter with a full scale error of 0.1 liters per, minute and a range of 1-10 liters per minute would generate the same error of 0.1 liters regardless of flow rate. This flow meter would be advantageous at higher flow rate values, because the fixed error is a smaller contribution to the level of inaccuracy. Since the error is a constant, the total error to the flow rate measured decreases as the flow rate increases is from 10% at 1 liter per minute, down to 1% at 10 liters per minute. As another example: a flow meter with a full scale error of 1% and a range of 1-10 liters per minute. As the flow rate increases on this flow meter, the level of inaccuracy remains a constant percentage of the flow rate. Regardless of the amount dispensed, the error remains a fixed percentage of the flow rate. As the fluid is dispensed through the flow meter to a vessel, the amount of inaccurate constituent is a product of the error rate, whether a fixed error rate or a fixed percentage of the flow rate, and the amount of time the fluid is dispensed through the flow meter. This contribution becomes a constant within a process, which is typically tuned in as the system operates.
In each case, the end result of the liquid chemical blend process is a volume of fluid maintaining a specified recipe. This recipe is detailed as a selection of ratios of a selection of certain chemical species, reagents, starting compounds, premixed ratios, and other liquid or powdered types. The requirements of the recipe is such that a certain level of tolerance must be maintained or the contents of the batch, with respect to the specific process, must be registered as unacceptable for use. An example of one such case is in the dilution of certain acids, more specifically hydrofluoric acid, used to etch layers of a wafer substrate. The etching to the layers is the result of a wafer immersed in a bath environment of hydrofluoric acid, which may be manipulated within the bath environment, such that a uniform removal of material is achieved. This process requires a specified concentration such that, over time, the material removal rate of wafers exposed is consistent: too low a concentration will not remove enough material, exposing the specific layer to be treated in successive process steps; too high a concentration may expose underlying surfaces resulting in a damaged wafer, only to be scrapped from the production lot. Another example relates to chemical and mechanical polishing, or planarization, of a wafer substrate involving metal contacts, such as tungsten or copper interconnections. An abrasive suspension involving an inert material, such as silicon dioxide or alumina oxide, is dispersed in a medium of de-ionized water containing additional modifying chemicals to maintain certain pH values for the removal process. These slurries may be mixed with oxidizing agents, such as hydrogen peroxide, ferric nitrate, potassium iodate, where the oxidizer specifically interacts with the metallic constituent of the substrate, through chemical bonding, oxidizing the metal ions. The inert abrasive acts as a mechanical agent, in conjunction with the polishing surface, to remove the oxidized metals, exposing fresh layers of material to be treated in the process. Again, the process is such that a specific concentration of oxidizer must be maintained to insure uniform removal rates, wafer to wafer, throughout the production process. If the oxidizer content varies, in the case of a below target oxidizer concentration, incomplete removal may ensue, leaving a partially planarized wafer; if the oxidizer concentration is above the specified target value, over-polished surfaces may expose and even, in some cases, damage the wafer, leading to a scrapped wafer.
Maintaining a constant supply of blended chemical to a process, where it is consumed in the production of semiconductor wafer substrates, is of importance when a fabrication facility is in full-scale production. The flow of wafers through a production facility is reduced where a bottleneck occurs, as in the case of any production facility. Bottlenecks within liquid chemical blend and dispense systems may arise from flow rate regulation, as a result of attempting to capture an accurate blend through regulating incoming liquid streams to a blending system. Factors that contribute to the duration of blending include addition of each stream to the system, circulation of product as modifying chemicals are added to the process, agitation of these constituents to achieve a desired uniform blend, measurement of the final blend, qualification procedures towards acceptance of the blend, subsequent additions to out-of-specification batches, circulation time prior to measurement, and requalification of the blend. In the event that a batch overshoots, some blending system architectures may not possess techniques to adjust a blend, leading to the contents being dumped to a drain line, only to restart the entire batch process. This loss of a batch leads to lost time and wasted chemical.
The addition of process materials to the mixing vessel is typically monitored and regulated by measuring mass or volume differences. Typical mass difference-regulated additions may involve the use of a scale on a holding vessel or tank. In this type of system, each process material is added individually, as an automated control system is not able to discern the relative amounts of two process materials added simultaneously. Typical volume difference regulated additions may involve the use of a flow meter. In either case, if the incoming liquids vary outside of the specified range, there is no immediate knowledge of an unacceptable blend until final measurements are made during acceptance qualification.
Many conventional processes require precise addition of process materials to produce a batch of blended process materials that is acceptable for its intended application. Accordingly, the measuring instruments that monitor the inputs to the mixing vessel are typically very precise to ensure batch-to-batch consistency. In most applications, even minor process variations may lead to significant differences in the batch of blended process materials, potentially rendering it useless for its intended application.
The full scale production of wafers incorporate manufacturing techniques, where semiconductor substrates and device wafers are exposed to steps; which include removal of unnecessary layers as a result of deposition steps, required planarization steps, and etching functions. These functions may require time-based processes where wafer substrates are exposed to solutions while immersed in a bath or sink, sprayed with chemical solutions during cleaning processes, undergo chemical and mechanical polishing sequences to remove material and prepare a surface for sequential process steps. Due to the high cost of producing these wafer substrates, the chemical solutions used require exacting accuracy for each blended batch of chemicals produced, in order to provide consistency in the manufacturing process. Batch to batch consistency must be maintained; insuring the overall process of wafer manufacturing minimizes scrapped wafers.
Multiple chemical recipes are becoming a prominent feature in system requirements for slurry, post CMP (Chemical Mechanical Planarization) clean, plating baths, developers, etc. The recipes required call for blending precision at or below (i.e., better than) 1%. These recipes may possess ratios from 1:1 down to 1:1000 in scale. Accurately specifying the system components necessary creates a task for any system design engineer.